I found that a warrior with sufficient AP to use protect cannot use protect if they do not have enough HP to survive the damage taken.
Let the warrior decide to make the ultimate sacrifice, but give them the classic dungeon master warning, "Are you SURE you want to do this?" first. This should cue the party to pre-heal the warrior or let them use their own first aid. But if they want to do it anyway, just let them. I know it's bad game play, but it's epic(ly) bad game play that the students will remember forever.